use std::{
    sync::{mpsc, Arc, Mutex},
    thread::{self, Thread}
};

pub struct ThreadPool{
    workers: Vec<Worker>,
    sender:mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;

impl ThreadPool {
    pub fn new(size: usize)-> ThreadPool {
        assert!(size > 0);

        let (sender, receiver) = mpsc::channel();
        let receiver = Arc::new(Mutex::new(receiver));
        let mut workers = Vec::with_capacity(size);
        for id in 0..size {
            workers.push(Worker::new(id, Arc::clone(&receiver)));
        }

        ThreadPool { workers, sender}

    }

    pub fn execute<F>(&self, f:F) 
    where F:FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(job).unwrap();
    }
}

impl Drop for ThreadPool  {
    fn drop(&mut self) {
        for worker in &mut self.workers.drain(..) {
            println!("Shutting down workder {}", worker.id);

            worker.thread.join().unwrap();
        }
    }
}

struct Worker {
    id: usize,
    thread: thread::JoinHandle<()>,
}

impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || {
            loop {
                let job = receiver.lock().unwrap().recv().unwrap();
                print!("Worker {id} got a job; executing.");
                job();
            }
        });
        Worker { id, thread }
    }
}